Hydrocracking of hydrocarbon feedstocks is often used to convert lower value hydrocarbon fractions into higher value products, such as conversion of vacuum gas oil (VGO) feedstocks to diesel fuel and lubricants. Typical hydrocracking reaction schemes can include an initial hydrotreatment step, a hydrocracking step, and a post hydrotreatment step. After these steps, the effluent can be fractionated to separate out a desired diesel fuel and/or lubricant oil basestock.
One way to improve the yield of a desired product is to use catalytic dewaxing to modify heavier molecules. Unfortunately, conventional methods for producing low pour point or low cloud point diesel fuel and/or lubricant oil basestock are hindered due to differing sensitivities for the catalysts involved in the various stages. This limits the selection of feeds which are potentially suitable for use in forming dewaxed diesel and/or Group II or higher basestocks. In conventional processing, the catalysts used for the hydroprocessing and hydrocracking of the oil fraction often have a relatively high tolerance for contaminants such as sulfur or nitrogen. By contrast, catalysts for catalytic dewaxing usually suffer from a low tolerance for contaminants. In particular, dewaxing catalysts that are selective for producing high yields of diesel and high yields and high VI lube oil and are intended to operate primarily by isomerization are typically quite sensitive to the amount of sulfur and/or nitrogen present in a feed. If contaminants are present, the activity, distillate selectivity and lubricating oil yield of the dewaxing catalyst will be reduced.
To accommodate the differing tolerances of the catalysts, a catalytic dewaxing step is often segregated from other hydroprocessing steps. In addition to requiring a separate reactor for the catalytic dewaxing, this segregation requires costly facilities and is inconvenient as it dictates the order of steps in the hydroprocessing sequence.
U.S. Pat. No. 8,231,778 describes a method for hydrocracking a feed to produce lubricant base oils and various fuels. The hydrocracking is performed in the presence of a catalyst composed of zeolite Beta, Y zeolite, or MFI zeolite. The metals supported on the catalyst can include non-noble Group VIII metals. It is noted that feeds can be hydrotreated prior to exposure to the hydrocracking catalyst in order to reduce the sulfur content of the feed to 500 wppm or less.
U.S. Patent Application Publication 2011/0315599 describes methods for using hydrocracking and dewaxing processes to produce lubricants and fuels. Various configurations are described for hydrotreating, hydrocracking, and/or dewaxing of feeds. Suitable hydrocracking catalysts are described as amorphous silica-alumina with supported metals, zeolite Y, or acidified alumina.
U.S. Pat. No. 6,884,339 describes a method for processing a feed to produce a lubricant base oil and optionally distillate products. A feed is hydrotreated and then hydrocracked without intermediate separation. An example of the catalyst for hydrocracking can be a supported Y or beta zeolite. The catalyst also includes a hydro-dehydrogenating metal, such as a combination of Ni and Mo. The hydrotreated, hydrocracked effluent is then atmospherically distilled. The portion boiling above 340° C. is catalytically dewaxed in the presence of a bound molecular sieve that includes a hydro-dehydrogenating element. The molecular sieve can be ZSM-48, EU-2, EU-11, or ZBM-30. The hydro-dehydrogenating element can be a noble Group VIII metal, such as Pt or Pd.
U.S. Pat. No. 7,300,900 describes a catalyst and a method for using the catalyst to perform conversion on a hydrocarbon feed. The catalyst includes both a Y zeolite and a zeolite selected from ZBM-30, ZSM-48, EU-2, and EU-11. Examples are provided of a two stage process, with a first stage hydrotreatment of a feed to reduce the sulfur content of the feed to 15 wppm, followed by hydroprocessing using the catalyst containing the two zeolites. An option is also described where it appears that the effluent from a hydrotreatment stage is cascaded without separation to the dual-zeolite catalyst, but no example is provided of the sulfur level of the initial feed for such a process.